net: check dead netns for peernet2id_alloc()

Andrei reports we still allocate netns ID from idr after we destroy
it in cleanup_net().

cleanup_net():
  ...
  idr_destroy(&net->netns_ids);
  ...
  list_for_each_entry_reverse(ops, &pernet_list, list)
    ops_exit_list(ops, &net_exit_list);
      -> rollback_registered_many()
        -> rtmsg_ifinfo_build_skb()
         -> rtnl_fill_ifinfo()
           -> peernet2id_alloc()

After that point we should not even access net->netns_ids, we
should check the death of the current netns as early as we can in
peernet2id_alloc().

For net-next we can consider to avoid sending rtmsg totally,
it is a good optimization for netns teardown path.

Fixes: 0c7aecd4bde4 ("netns: add rtnl cmd to add and get peer netns ids")
Reported-by: Andrei Vagin <avagin@gmail.com>
Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Andrei Vagin <avagin@openvz.org>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -219,6 +219,8 @@ int peernet2id_alloc(struct net *net, struct net *peer)
 	bool alloc;
 	int id;
 
+	if (atomic_read(&net->count) == 0)
+		return NETNSA_NSID_NOT_ASSIGNED;
 	spin_lock_irqsave(&net->nsid_lock, flags);
 	alloc = atomic_read(&peer->count) == 0 ? false : true;
 	id = __peernet2id_alloc(net, peer, &alloc);